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fn9208 rev 4.00 page 1 of 28 sep 30, 2008 fn9208 rev 4.00 sep 30, 2008 isl6308 three-phase buck pwm controller with high current integrated mo sfet drivers datasheet the isl6308 is a three-ph ase pwm control ic with integrated mosfet drivers. it p rovides a precision voltage regulation system for multiple a pplications including, but not limited to, high current low vol tage point-of-load converters, embedded applications and o ther general purpose low voltage medium to high curren t applications.the integration of power mosfet drivers into the controller ic marks a departure from the separate pwm controller and driver configuration of previous mult i-phase product families. by reducing the number of external p arts, this integration allows for a cost and space saving power management solution. output voltage can be programmed using the on-chip dac or an external preci sion reference. a 2- bit code programs the dac reference to one o f 4 possible values (0.6v, 0.9v,1.2v and 1.5v). a unity gain, differ ential amplifier is provided for remote voltage s ensing, compensating for any potential difference between r emote and local grounds. the output voltage can also be offs et through the use of single external resistor. an opti onal droop function is also implemented and can be disabl ed for applications having less stringent output voltage variation requirements or experiencing less severe step loads. a unique feature of the isl6308 is the combined use of both dcr and r ds(on) current sensing. load line voltage positioning and overcurrent p rotection are accomplished through continuous inductor dcr current sensing, while r ds(on) current sensing is used for accurate channel-current balance. using both methods of c urrent sampling utilizes the best advantages of each technique. protection features of this c ontroller ic include a set of sophisticated overvoltage a nd overcurrent protection. overvoltage results in the c onverter turning the lower mosfets on to clamp the rising output voltage and protect the load. an ovp output is also provided to drive an optional crowbar device. the overcurrent protection level is set through a single external resistor. other protection features include protection against an open circuit on the remote sensing inputs. combined, these features provide advanced protection for the output load. features ? integrated multi-phase power conversion - 1, 2, or 3-phase operation ? precision output voltage regulation - differential remote voltage sensing - 0.8% system accuracy over-temperature (for ref = 0.6v and 0.9v) - 0.5% system accuracy over-temperature (for ref = 1.2v and 1.5v) - usable for output volta ges not exceeding 2.3v - adjustable reference-voltage offset ? precision channel current sharing - uses loss-less r ds(on) current sampling ? optional load line (droop) programming - uses loss-less inductor dcr current sampling ? variable gate-drive bias - 5v to 12v ? internal or external reference voltage setting - on-chip adjustable fixed dac reference voltage with 2-bit logic input selects f rom four fixed reference voltages (0.6v, 0.9v, 1.2v, 1.5v) - reference can be changed dynamically - can use an external voltage reference ? overcurrent protection ? multi-tiered overvoltage protection - ovp pin to drive optional crowbar device ? selectable operation frequ ency up to 1.5mh z per phase ? digital soft-start ? capable of start-up in a pre-biased load ? pb-free (rohs compliant) applications ? high current ddr/chipset core voltage regulators ? high current, low voltage dc/dc converters ? high current, low voltage fpga/asic dc/dc converters n o t r e c o m m e n d e d f o r n e w d e s i g n s r e c o m m e n d e d r e p l a c e m e n t p a r t i s l 6 3 0 8 a
isl6308 fn9208 rev 4.00 page 2 of 28 sep 30, 2008 pinout isl6308 (40 ld 6x6 qfn) top view ordering information part number (note) part marking temp. range (c) package (pb-free) pkg. dwg. # isl6308crz isl6308 crz 0 to +70 40 ld 6x6 qfn l40.6x6 isl6308crz-t* isl6308 crz 0 to +70 40 ld 6x6 qfn l40.6x6 isl6308irz isl6308 irz -40 to +85 40 ld 6x6 qfn l40.6x6 isl6308irz-t* isl6308 irz -40 to +85 40 ld 6x6 qfn l40.6x6 ISL6308EVAL1Z evaluation platform * please refer to tb347 for details on reel specifications. note: these intersil pb-free plastic packaged products employ sp ecial pb-free material sets, molding compounds/die attach mater ials, and 100% matte tin plate plus anneal (e3 termination finish, which is ro hs compliant and compatible with both snpb and pb-free solderin g operations). intersil pb-free products are msl classified at pb-free peak re flow temperatures that meet or exceed the pb-free requirements of ipc/jedec j std-020. 3ph vsen ref1 ovp enll pgood lgate1 boot1 ref0 fs pvcc1 isen1 ugate1 phase1 phase2 ugate2 boot2 isen2 pvcc2 lgate2 phase3 boot3 ocset icomp droop isum iref lgate3 pvcc3 isen3 ugate3 2ph dac ref ofst vcc comp fb vdiff rgnd 1 40 2 3 4 5 6 7 8 9 10 30 29 28 27 26 25 24 23 22 21 39 38 37 36 35 34 33 32 31 11 12 13 14 15 16 17 18 19 20 41 gnd
isl6308 fn9208 rev 4.00 page 3 of 28 sep 30, 2008 block diagram dac dac ref1 ref0 e/a ref fb offset ofst comp isum iref icomp isen amp oc ocset rgnd vsen vdiff 100a +150mv ovp x 0.82 ovp uvp isen1 isen2 isen3 channel current sense ? 1 n ? pwm1 ? ? pwm3 channel current balance boot1 ugate1 phase1 lgate1 pvcc1 through shoot- protection boot2 ugate2 phase2 lgate2 pvcc2 logic control gate through shoot- protection boot3 ugate3 phase3 lgate3 pvcc3 logic control gate soft-start and fault logic channel detect vcc reset power-on 0.66v enll fs pgood gnd 0.2v +1v pwm2 droop ovp 3ph 2ph x1 x1 clock and generator sawtooth through shoot- protection logic control gate
isl6308 fn9208 rev 4.00 page 4 of 28 sep 30, 2008 typical application - isl6308 pgood vdiff fb comp vcc isen1 ref1 fs ofst ref +12v +12v +12v phase1 ugate1 boot1 lgate1 isen2 phase2 ugate2 boot2 lgate2 isen3 phase3 ugate3 boot3 lgate3 isum icomp iref load vsen rgnd ocset ref0 +5v pvcc1 pvcc2 pvcc3 enll +12v gnd ovp 2ph 3ph dac droop isl6308
isl6308 fn9208 rev 4.00 page 5 of 28 sep 30, 2008 absolute maximum ratings supply voltage, vcc . . . . . . . . . . . . . . . . . . . . . . . . . . . -0.3v to +6v supply voltage, pvcc . . . . . . . . . . . . . . . . . . . . . . . . . -0.3v to +15v absolute boot voltage, v boot . . . . . . . . gnd - 0.3v to gnd + 36v phase voltage, v phase . . . . . . . . gnd - 0.3v to 15v (pvcc = 12) gnd - 8v (<400ns, 20j) to 24v (<200ns, v boot-phase = 12v) upper gate voltage, v ugate . . . . v phase - 0.3v to v boot + 0.3v v phase - 3.5v (<100ns pulse width, 2j) to v boot + 0.3v lower gate voltage, v lgate . . . . . . . . gnd - 0.3v to pvcc + 0.3v gnd - 5v (<100ns pulse width, 2j) to pvcc+ 0.3v input, output, or i/o voltage . . . . . . . . . gnd - 0.3v to v cc + 0.3v thermal information thermal resistance (typical notes 1, 2) ? ja (c/w) ? jc (c/w) 40 ld qfn package . . . . . . . . . . . . . . 32 3.5 maximum junction temperature . . . . . . . . . . . . . . . . . . . . . . +150c maximum storage temperature range . . . . . . . . . .-65c to +1 50c pb-free reflow profile. . . . . . . . . . . . . . . . . . . . . . . . .see link below http://www.intersil.com/pbfree/pb-freereflow.asp recommended operating conditions vcc supply voltage . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .+5v ? 5% pvcc supply voltage . . . . . . . . . . . . . . . . . . . . . . . +5v to 12v ? 5% ambient temperature (isl6308crz) . . . . . . . . . . . . . 0c to +70c ambient temperature (isl6308irz) . . . . . . . . . . . . .-40c to +85c caution: do not operate at or near the maximum ratings listed fo r extended periods of time. exposure to such conditions may adv ersely impact product reliability and result in failures not covered by warranty. notes: 1. ? ja is measured in free air with the component mounted on a high e ffective thermal conductivity t est board with direct attach f eatures. see tech brief tb379. 2. for ? jc , the case temp location is the center of the exposed metal p ad on the package underside. electrical specifications recommended operating conditions , unless otherwise specified. p arameters with min and/or max limits are 100% tested at +25c, unless otherwise specified. temperature l imits established by c haracterization and are not production tested. parameter test conditions min typ max units bias supply and internal oscillator input bias supply current i vcc ; enll = high 12 15 18 ma gate drive bias current i pvcc ; enll = high; all gate outputs open, f sw = 250khz 0.4 0.8 1.2 ma vcc por (power-on reset) threshold vcc rising 4.25 4.38 4.47 v vcc falling 3.75 3.88 4.00 v pvcc por (power-on reset) threshold pvcc rising 4.25 4.38 4.47 v pvcc falling 3.75 3.88 4.00 v oscillator frequency f sw 225 250 275 khz oscillator ramp amplitude (note 3) v p-p -1.50-v maximum duty cycle (note 3) -66.6-% control thresholds enll rising threshold 0.595 0.630 0.670 v enll hysteresis 54 61 68 mv enll pull-up current enll = 1v -0.5 -1.4 -3.0 a comp shutdown threshold comp falling 0.1 0.25 0.4 v reference and dac system accuracy (dac = 0.6v, 0.9v) droop connected to iref -0.8 - 0 .8 % system accuracy (dac = 1.2v, 1.50v) droop connected to iref -0.5 - 0.5 % dac input low voltage (ref0, ref1) --0.4v dac input high voltage (ref0, ref1) 0.8 - - v minimum external reference (note 4) ref pin input - 0.6 - v maximum external reference (note 4) ref pin input - 1.75 - v ofs sink current accuracy (negative offset) r ofs = 30k ?? from ofs to vcc 47.5 50.0 52.5 a ofs source current accuracy (positive offset) r ofs = 10k ?? from ofs to gnd 47.5 50.0 52.5 a
isl6308 fn9208 rev 4.00 page 6 of 28 sep 30, 2008 error amplifier dc gain (note 3) r l = 10k to ground - 96 - db gain-bandwidth product (note 3) c l = 100pf, r l = 10k to ground - 20 - mhz slew rate (note 3) c l = 100pf, load = 400a - 8 - v/s maximum output voltage vcc = 5v; load = 1ma 3.90 4.20 4.60 v minimum output voltage load = -1ma 0.6 0.85 1.0 v remote sense differential amplifier input bias current (vsen) (vsen = 1.5v) 49 55 60 a bandwidth (note 3) -20-mhz slew rate (note 3) -8-v/s overcurrent protection ocset trip current 93 100 107 a ocset accuracy oc comparator offs et (ocset and isum difference) - 50 5mv icomp offset isen amplifier offset -5 0 5 mv protection undervoltage threshold vsen falling 80 82 84 %vid undervoltage hysteresis vsen rising - vsen falling 2 3 4 %vid overvoltage threshold while ic disabled 1.62 1.67 1.72 v overvoltage threshold vsen rising dac + 125mv dac + 150mv dac + 175mv v overvoltage hysteresis vsen rising - vsen falling 75 100 125 mv open sense-line protection thres hold iref rising and falling vdif f + 0.9v vdiff + 1v vdiff + 1.1v v ovp output high drive voltage i ovp = 50ma, vcc = 5v 2.2 3.8 4.4 v pgood output low vcc = 5v; 1k ? resistor to vcc 0 0.2 0.4 v switching time ugate rise time (note 3) t rugate; v pvcc = 12v, 3nf load, 10% to 90% - 26 - ns lgate rise time (note 3) t rlgate; v pvcc = 12v, 3nf load, 10% to 90% - 18 - ns ugate fall time (note 3) t fugate; v pvcc = 12v, 3nf load, 90% to 10% - 18 - ns lgate fall time (note 3) t flgate; v pvcc = 12v, 3nf load, 90% to 10% - 12 - ns ugate turn-on non-overlap (note 3) t pdhugate ; v pvcc = 12v, 3nf load, adaptive - 10 - ns lgate turn-on non-overlap (note 3) t pdhlgate ; v pvcc = 12v, 3nf load, adaptive - 10 - ns gate drive resistance (note 4) upper drive source resistance v pvcc = 12v, 150ma source current 1.25 2.0 3.0 ? upper drive sink resistance v pvcc = 12v, 150ma sink current 0.9 1.6 3.0 ? lower drive source resistance v pvcc = 12v, 150ma source current 0.85 1.4 2.2 ? lower drive sink resistance v pvcc = 12v, 150ma sink current 0.60 0.94 1.35 ? over-temperature shutdown thermal shutdown setpoint (note 4) - 160 - c thermal recovery setpoint (note 4) - 100 - c notes: 3. limits should be considered ty pical and are not production te sted. 4. limits established by characteri zation and are not production tested. electrical specifications recommended operating conditions , unless otherwise specified. p arameters with min and/or max limits are 100% tested at +25c, unless otherwise specified. temperature l imits established by c haracterization and are not production tested. (continued) parameter test conditions min typ max units
isl6308 fn9208 rev 4.00 page 7 of 28 sep 30, 2008 timing diagram simplified power system diagram functional pin description vcc (pin 6) bias supply for the ics small-si gnal circuitry. connect this pin to a +5v supply and loca lly decouple using a quality 1.0f ceramic capacitor. pvcc1, pvcc2, pvcc3 (pins 33, 24, 18) power supply pins for the corresponding channel mosfet drive. these pins can be connected to any voltage from +5v to +12v, depending on the desired mosfet gate drive level. note that tying pvcc2 or pvcc3 to gnd has the same effect as tying 2ph or 3ph to gnd for disabling the corresponding phase gnd (pin 41) bias and reference g round for the ic. enll (pin 37) this pin is a threshold sensitiv e (approximately 0.66v) enable input for the controller. held l ow, this pin disab les controlle r operation. pulled high, the pin enables the controller for operation. fs (pin 36) a resistor, placed from fs to g round, will set the switching frequency. refer to equation 43 and figure 23 for proper resistor calculation. 3ph and 2ph (pins 1, 2) these pins decide how many phases the controller will operate. tying both pins to vcc allows for 3-phase operation. tying the 3ph pin to gnd causes the controller to operate in 2-phase mode, while connecting b oth 3ph and 2ph gnd will allow for single phase operation. ugate lgate t flgate t pdhugate t rugate t fugate t pdhlgate t rlgate channel1 +5v in v out q1 q2 isl6308 channel2 q3 q4 dac channel3 q5 q6 +12v in enll pgood 2 ref0,ref1 ovp
isl6308 fn9208 rev 4.00 page 8 of 28 sep 30, 2008 ref0 and ref1 (pins 40, 39) these pins make up t he 2-bit input that selects the fixed dac reference voltage. thes e pins respond to ttl logic thresholds. the isl6308 deco des these inputs to establish one of four fixed reference voltages; see table 1 for correspondence between ref0 and ref1 inputs and reference voltage settings. these pins are internally pulled high, to approximately 1.2v, by 40a (typically) internal current sources; the internal pull-up current decreases to 0 as the ref0 and ref1 voltages approach the internal pull-up voltage. both ref0 and ref1 pins are compatible with external pull-up voltages not exceeding the ics bias voltage (vcc). rgnd and vsen (pins 10, 11) rgnd and vsen are inputs to th e precision differential remote-sense amplifier and s hould be connected to the sense pins of the remote load. icomp, isum, and iref (pins 13, 15, 16) isum, iref, and icomp ar e the dcr current sense amplifiers negative input, positive input, and output respectively. for accurate dcr current sensing, connect a resistor from each channels phase node to isum and connect iref to the summing point of the output inductors. a parallel r-c feedback cir cuit connected between isum and icomp will then create a voltage fr om iref to icomp proportional to the voltage d rop across the inductor dcr. this voltage is referred to as the droop voltage and is added to the differential remote -sense amplifiers output. an optional 0.001f to 0.01f ceramic capacitor can be placed from the iref pin to the isum pin t o help reduce common mode noise that might be introduc ed by the layout. droop (pin 14) this pin enables or disables droop. tie this pin to the icomp pin to enable droop. to disable droop, tie this pin to the iref pin. vdiff (pin 9) vdiff is the output of the differential remote-sense amplifier. the voltage on this pi n is equal to the difference between vsen and rgnd added to the difference between iref and icomp. vdiff ther efore represents the vout voltage plus the droop voltage. comp and fb (pins 7, 8) the internal error amplifiers inverting input and output respectively. fb is connected to vdiff through an external r or r-c network depending on the desired type of compensation (type ii or iii). comp is tied back to fb through an external r-c network to compensate the regulator. dac (pin 3) the dac pin is the direct output of the internal dac. this pin is connected to the ref pin using a 1k ? to 5k ? resistor. this pin can be left open if an external reference is used. ref (pin 4) the ref input pin is the positiv e input of the error amplifier. this pin can be connected to t he dac pin using a resistor (1k ? to 5k ? ) when the internal dac v oltage is used as the reference voltage. when an ex ternal voltage reference is used, it must be connected dire ctly to the ref pin, while the dac pin is left unconnected. the output voltage will be regulated to the voltage at the ref pin unless this voltage is greater than the voltage at the dac pin. if an external reference is used at this pin, its magnitude cannot exceed 1.75v. a capacitor is used between the ref pin and ground to smooth the dac voltage during soft-start. ofst (pin 5) the ofst pin provides a means to program a dc current for generating an offset voltage ac ross the resistor between fb and vdiff. the offset current is generated via an external resistor and preci sion internal voltage references. the polarity of the offset is select ed by connecting the resistor t o gnd or vcc. for no offset, th e ofst pin should be left unconnected. ocset (pin 12) this is the overcurrent set pin. placing a resistor from ocset to icomp, allows a 100a curr ent to flow out of this pin, producing a voltage reference. i nternal circuitry compares the voltage at ocset to the voltage at isum, and if isum ever exceeds ocset, the overcurr ent protection activates. isen1, isen2 and isen3 (pins 32, 25, 19) these pins are used for balanc ing the channel currents by sensing the current through each channels lower mosfet when it is conducting. connec t a resistor between the isen1, isen2, and isen3 pins and their respective phase node. this resistor sets a curr ent proportional to the current in the lower mosfet during its conduction interval. ugate1, ugate2, and ugate3 (pins 31, 27, 20) connect these pins to the upper mosfets gates. these pins are used to control t he upper mosfets and are monitored for shoot-through prevention purposes. maximum individual channel duty cycle is limited to 66%. boot1, boot2, and boot3 (pins 30, 26, 21) these pins provide the bias voltage for the upper mosfets drives. connect these pins to a ppropriately-chosen external bootstrap capacitors. internal bootstrap diodes connected to the pvcc pins provide the nec essary bootstrap charge.
isl6308 fn9208 rev 4.00 page 9 of 28 sep 30, 2008 phase1, phase2, and phase3 (pins 29, 28, 22) connect these pins to the so urces of the upper mosfets. these pins are the return path for the upper mosfets drives. lgate1, lgate2, and lgate3 (pins 34, 23, 17) these pins are used to control the lower mosfets and are monitored for shoot-through prevention purposes. connect these pins to the lower mosfets gates. do not use external series gate resistors as this might lead to shoot-through. pgood (pin 35) pgood is used as an indication of the end of soft-start. it is an open-drain logic ou tput that is low i mpedance until the soft-start is completed and vout is equal to the vid setting. once in normal operation, pgo od indicates whether the output voltage is within specified overvoltage and undervoltage limits. if the output voltage exceeds these limits or a reset event occurs (such as an overcurrent event), pgood becomes high impedance again. the potential at this pin should not exceed that of the potential at vcc pin by more than a typical forwar d diode drop at any time. ovp (pin 38) overvoltage protection pin. this pin pulls to vcc when an overvoltage conditi on is detect ed. connect this pin to the gate of an scr or mos fet tied across v in and ground to prevent damage to a load device. operation multi-phase power conversion modern low voltage dc/dc converter load current profiles have changed to the point that the advantages of multi-phase power conversion are impossible to ignore. the technical challenges associated with producing a single-phase converter that is both cost-eff ective and thermally viable have forced a change to the cost-sav ing approach of multi-phase. the isl6308 controller helps s implify implementation by integrating vital functions and requiring minimal output components. the block diagr am on page 3 provides a top level view of multi-phase power conversion using the isl6308 controller. interleaving the switching of each channel i n a multi-phase converter is timed to be symmetrically out -of-phase with each of the other channels. in a 3-phase converter, each channel switches one t hird cycle after the pr evious chan nel and one third cycle before t he following channel. as a result, the three-phase converter has a combined ripple frequency three times greater than the ripple frequency of any one phase. in addition, the peak -to-peak amplitude of the combined inductor currents is reduced in proportion to the number of phases (equations 1 and 2). increased ripple frequency and lower ripple amplitude mean that the designer can use less per-channel induc tance and lower total output capacitance for any perf ormance specification. figure 1 illustrates the multip licative effect o n output ripple frequency. the three channel currents (i l1 , i l2 , and i l3 ) combine to form the ac rip ple current and the dc load current. the ripple component has three times the ripple frequency of each individual channel current. each pwm pulse is terminated one third of a cycle after the pwm pulse of the previous phase. the peak-to -peak current for each phase is about 7a, and the dc components of the inductor currents combine to feed the load. to understand the reduction of ripple current amplitude in the multi-phase circuit, examine equation 1, which represents an individual channel peak -to-peak inductor current. in equation 1, v in and v out are the input and output voltages respectively, l is the single-channel inductor value, and f sw is the switching frequency. the output capacitors conduct the ripple component of the inductor current. in the case of multi-phase converters, the capacitor current is the sum of the ripple currents from each of the individual channels. compare equat ion 1 to the expression for the peak-to-peak current after the summation of n symmetrically phase-shi fted inductor currents in equation 2. peak-to-peak rippl e current decreases by an amount proportional to the number of channels. output voltage ripple is a function o f capacitance, capacitor equivalent series resistance (esr), and inductor ripple current. reducing the inductor ripple current allows the designer to use fewer or less costly output capacitors. figure 1. pwm and inductor-current waveforms for 3-phase converter pwm2, 5v/div pwm1, 5v/div i l2 , 7a/div i l1 , 7a/div i l1 + i l2 + i l3 , 7a/div i l3 , 7a/div pwm3, 5v/div i pp v in v out C ?? v out ? lf sw v in ?? --------------------------------------------------------- - = (eq. 1) i cpp , v in nv out ? C ?? v out ? lf sw v ? in ? --------------------------------------------------------------- ---- - = (eq. 2)
isl6308 fn9208 rev 4.00 page 10 of 28 sep 30, 2008 another benefit of interleavin g is to reduce input ripple current. input capacitance is determined in part by the maximum input ripple current. multiphase topologies can improve overall system cost and size by lowering input ripple current and allowing the designer to reduce the cost of input capacitance. the example in f igure 2 illustrates input currents from a three-phase c onverter combining to reduce the total input ripple current. the converter depicted in figure 2 delivers 1.5v to a 36a load from a 12v input. the rms input capacitor current is 6.1a. compare this to a single-phase converter also stepping down 12v to 1.5v at 36a. the single -phase converter has a 13.3a rms input capacitor current . the single-phase converter must use an input capacitor bank with twice the rms current capacity as the equivalent three-phase converter. figures 24, 25 and 26 in the sect ion entitled input capacitor selection on page 25 can be used to determi ne the input capacitor rms current based on load current, duty cycle, and the number of channels. th ey are provided as aids in determining the optimal inpu t capacitor solution. pwm operation the timing of each converter leg is set by the number of active channels. the default channel setting for the isl6308 is three. one switching cycle is defin ed as the time between the internal pwm1 pulse termination signals. the pulse termination signal is the inte rnally generated clock signal that triggers the fa lling edge of pwm 1. the cycle time of the pulse termination signal is th e inverse of the switching frequency set by the resistor between the fs pin and ground. each cycle begins when the clock signal commands pwm1 to go low. the pwm1 transition signals the internal channel 1 mosfet driver to turn off the channel 1 upper mosfet and turn on the channel 1 synchronous mosfet. in the default channel configuration, the pwm2 pulse terminates one third of a cycle after t he pwm1 pulse. the pwm3 pulse terminates one th ird of a cycle after pwm2. if pvcc3 is left open or conne cted to ground, two channel operation is selected and t he pwm2 pulse terminates one half of a cycle after the pwm1 pulse terminates. if both pvcc3 and pvcc2 are left op en or connected to ground, single channel operation is selected. the 2ph and 3ph inputs can also be used to acco mplish thi s function. once a pwm pulse transitions low, it i s held low for a minimum of one third cycle. this forced off time is re quired to ensure an accurate current sample. current sensing is described in the next section. after the force d off time expires, the pwm output is enabled. the pwm out put state is driven by the position of the error amplifier output signal, vcomp, minus the current correction signal re lative to the sawtooth ramp as illustrated in figure 3. when the modified vcomp voltage crosses the sawtooth ramp, the pwm output transitions high. the internal mosfet dr iver detects t he change in state of the pwm signal and turns off the synchronous mosfet and turns on the upper mosfet. the pwm signal will remain high until the pulse terminati on signal marks the beginning of the next cycle by triggering the pwm signal low. channel current balance one important benefit of multi- phase operation is the thermal advantage gained by distributi ng the dissipated heat over multiple devices and greater ar ea. by doing this the designer avoids the complexity of dri ving parallel mosfets and the expense of using exp ensive heat sinks a nd exotic magnetic materials. in order to realize the thermal advantage, it is important that each channel in a multi-phase c onverter be controlled to carry about the same amount of current at any load level. to achieve this, the currents th rough each channel must be sampled every switching c ycle. the sampled currents, i n , from each active channel are summed together and divided by the number of active channels. the resulting cycle average current, i avg , provides a measure o f the total load current demand on the conver ter during each switching cycle. channel current balance is achieved by comparing the sampled current of each channel to the cycle average current, and making the proper adjus tment to each channel pulse width based on the error. inters ils patented current-balance method is illustrated in figure 3, with error correction for channel 1 represented. in figure 3, the cycle average current, i avg , is compared with the channel 1 sample, i 1 , to create an error signal i er . the filtered error signal modifies the pulse width commanded by v comp to correct any unbalance and force i er toward zero. the same method for error signal correction is applied to each active channel. figure 2. channel input currents and input- capacitor rms current for 3-phase converter channel 2 input current channel 3 input current channel 1 input current input-capacitor current
isl6308 fn9208 rev 4.00 page 11 of 28 sep 30, 2008 current sampling in order to realize proper cur rent balance, the currents in each channel must be sampled every sw itching cycle. this sampling occurs during the forced off-time, following a pwm transition low. during this ti me the current sense amplifier uses the isen inputs to reprod uce a signal proportional to the inductor current, i l . this sensed current, i sen , is simply a scaled version of the inductor current. the sample window opens exactly one sixth of the switching period, t sw , after the pwm transitions low. t he sample window then stays open the rest of th e switching cycle unt il pwm transitions high again, as illust rated in figure 4. the sampled current, a t the end of the t sample , is proportional to the inductor curre nt and is held until the next switching period sample. the sampled current is used only for channel current balance. the isl6308 sup ports mosfet r ds(on) current sensing to sample each channels current for channel current balance. the internal circuitry, shown in figure 5 represents channel n of an n-channel converter. t his circuitry is repeated for each channel in the converte r, but may not be active depending on the status of the pvcc3 and pvcc2 pins, as described in pwm operation on page 10. the isl6308 senses the channe l load current by sampling the voltage across the lower mosfet r ds(on) , as shown in figure 5. a ground-referenced operational amplifier, internal to the isl6308, is connected to the ph ase node through a resistor, r isen . the voltage across r isen is equivalent to the voltage drop across the r ds(on) of the lower mosfet while it is conducting. the resu lting current into the isen pin is proportional to the channel current, i l . the isen current is sampled and held as describ ed in current sampling on page 11. from figure 5, equation 3 for i n is derived where i l is the channel current. output voltage setting the isl6308 uses a digital to analog converter (dac) to generate a reference voltage based on the logic signals at the ref1, ref0 pins. the dac decodes the 2-bit logic signals into one of the discrete voltages shown in table 1. each ref0 and ref1 pins are pulled up to an internal 1.2v voltage by weak current sources (40a current, decreasing to 0 as the voltage at the ref0, ref1 pins varies from 0 to the internal 1.2v pull-up voltage). external pull-up resistors or active-hig h output stages can augment the pul l-up current sources, up to a voltage of 5v. the dac pin must be connected to ref pin through a 1k ? to 5k ? resistor and a filter capacitor (0.022f) is connected between ref and gnd. the isl6308 accommodates the use of external voltage reference connected to ref pin if a different output voltage is required. the dac voltage must be set at least as high as the external reference. the erro r amp internal non-inverting input is the lower of ref or (dac +300mv). a third method for s etting the output voltage is to use a resistor divider (r p1 , r s1 ) from the output terminal (v out ) figure 3. channel 1 pwm function and current- balance adjustment ?? n i avg i 3 i 2 ? - + + - + - f(s) pwm1 i 1 v comp sawtooth signal i er note: channel 2 and 3 are optional. filter to gate control logic figure 4. sample and hold timing time pwm i l i sen switching period sampling period old sample current new sample current figure 5. isl6308 internal and external current- sensing circuitry for current balance i n i sen i l x r ds on ?? r isen ------------------------- - = - + isen(n) r isen sample and hold isl6308 internal circuit external circuit v in channel n upper mosfet channel n lower mosfet - + i l x r ds on ?? i l i n i l r ds on ?? r isen ---------------------- ? = (eq. 3)
isl6308 fn9208 rev 4.00 page 12 of 28 sep 30, 2008 to vsen pin to set the output voltage level as shown in figure 6. this method is good fo r generating voltages up to 2.3v (with the ref voltage set to 1.5v). for this case, the output volta ge can be obtained as follows: it is recommended to choose resistor values of less than 500 ? for r s1 and r p1 resistors in order t o get better output voltage dc accuracy. voltage regulation in order to regulate the output voltage to a specified level, t he isl6308 uses the integrating compensation network shown in figure 6. this compensation network insures that the steady state error in the output volt age is limited only to the error in the reference voltage (output of the dac or the external voltage reference) and offse t errors in the ofs current source, remote sense and error am plifiers. intersil specifies the guaranteed tolerance of the isl6308 to include the combined tolerances of each of these elements, except when an external reference or volt age divider is used, then the tolerances of these components has to be taken into account. the isl6308 incorporates an in ternal differential remote sense amplifier in the feedback path. the amplifier removes the voltage error encountered when measuring the output voltage relative to the cont roller ground reference point, resulting in a more accurate means of sensing output voltage. connect the microproces sor sense pins to the non-inverting input, vsen, and inverting input, rgnd, of the remote-sense amplifier. the droop voltage, vdroop, also feeds into the remote-sense amplifier. the re mote-sense output, vdiff, is therefore equal to the sum of t he output voltage, vout, and the droop voltage. vdiff is connected to the inverting input of the error amplifier through an external resistor. the output of the error amplifier, v comp , is compared to the sawtooth waveform to generate the pwm signals. the pwm signals control the timing of the i nternal mosfet drivers and regulate the converter output so that the voltage at fb is equal to the voltage at ref. t his will regulat e the output voltage to be equal to equation 5. the internal and external circuitry that controls voltage regulation is illustrated in figure 6. table 1. isl6308 dac voltage selection table ref1 ref0 vda (v) 0 0 0.600 0 1 0.900 1 0 1.200 1 1 1.500 v out v ref r s1 r p1 + ?? r p1 --------------------------------- - v ofs v droop C ? ? ? = (eq. 4) v out v ref v ofst ? v droop C = (eq. 5) figure 6. output voltage and load-line regulation with offset adjustment i ofs external circuit isl6308 internal circuit comp r 2 r 1 fb vdiff vsen rgnd - + v ofs error amplifier - + differential remote-sense amplifier v comp c 1 ref c ref - + vid dac iref droop + - + v droop - + v out - dac icomp + - isum isense amp r p1 r s1 c sum
isl6308 fn9208 rev 4.00 page 13 of 28 sep 30, 2008 load line (droop) regulation in some high current applicat ions, a requ irement on a precisely controlled output i mpedance is imposed. this dependence of output voltage on load current is often termed droop or load line regulation. the droop is an opti onal feature in the isl6308. it can be enabled by connecting icomp pin to droop pin, as shown in figure 6. to disable it, c onnect the droop pin to iref pin. as shown in figure 6, a voltage, v droop , proportional to the total current in all active channels, i out , feeds into the differential remote- sense amplifier. the resulting voltage at the output of the remote-sense amplifier is the sum of the output voltage and th e droop voltage. as equation 4 shows, feeding this voltage into the co mpensation network causes the regulator to adjust the out put voltage so that its equal t o the reference voltage mi nus the droop voltage. the droop voltage, v droop , is created by sensing the current through the output inductors. this is accomplished by using a continuous dcr current sensing method. inductor windings have a characteristic distributed resistance or dcr (direct current resistance). for simplicity, the inductor dcr is considered as a separate lumped quantity, as shown in figure 7. the channel current, i l , flowing through the inductor , passes through the dcr. equation 6 shows the s-domain equivalent voltage, v l , across the inductor. the inductor dcr is important because the voltage dropped across it is proportional to t he channel current. by using a simple r-c network and a current sense amplifier, as shown in figure 7, the voltage drop across all of the inductors dcrs can be extracted. the output of the current sense amplifier, v droop , can be shown to be pr oportional to the channel currents i l1 , i l2 , and i l3 , shown in equation 7. if the r-c network component s are selected such that the r-c time constant matches the inductor l/dcr time constant, then v droop is equal to the sum of the voltage drops across the individual dcrs , multiplied by a gain. as equation 8 shows, v droop is therefore proportional to the total output current, i out . by simply adjusting the value of r s , the load line can be set to any level, giving the converte r the right amount of droop at all load currents. it may also be necessary to compensate for any changes in dcr due to tem perature. these changes cause the load line to be skew ed, and cause the r-c time constant to not match the l/dcr time constant. if this becomes a problem a simple negative temperature coefficient resistor network can be used in the place of r comp to compensate for the rise in dcr due to temperature. output voltage offset programming the isl6308 allows the desig ner to accurately adjust the offset voltage by connecting a resistor, r ofs , from the ofs pin to vcc or gnd. when r ofs is connected between ofs and vcc, the voltage across i t is regulated to 1.5v. this causes a proportional current (i ofs ) to flow into the ofs pin and out of the fb pin. if r ofs is connected to ground, the voltage across it is regulated to 0.5v, and i ofs flows into the fb pin and out of the ofs pin . the offset current flowing through the resistor between vdiff and fb will generate the desired offset voltage which is equal to the product (i ofs xr 1 ). these functions are s hown in figures 8 and 9. v l s ?? i l sl dcr + ? ?? ? = (eq. 6) v droop s ?? sl ? dcr ------------- 1 + ?? ?? sr comp c comp ?? 1 + ?? --------------------------------------------------------------- ----------- r comp r s ----------------------- i l1 i l2 i l3 ++ ?? dcr ?? ? = (eq. 7) v droop r comp r s --------------------- i out dcr ?? = (eq. 8) figure 7. dcr sensing configuration - + icomp dcr l1 inductor v out c out i l 1 - + v l (s) dcr l2 inductor phase1 phase2 i l 2 r s r s r comp c comp isum iref isl6308 - + v droop i out droop c sum (optional)
isl6308 fn9208 rev 4.00 page 14 of 28 sep 30, 2008 once the desired output offset voltage has been determined, use the following formulas to set r ofs : for positive offset (connect r ofs to gnd): for negative offset (connect r ofs to vcc): advanced adaptive zero shoot-through deadtime control (patent pending) the integrated drivers inc orporate a unique adaptive deadtime control technique to m inimize deadtime, resulting in high efficiency from the red uced freewheeling time of the lower mosfet body-diode cond uction, and to prevent the upper and lower mosfets from conducting simultaneously. this is accomplished by ensurin g either rising gate turns on its mosfet with minimum and sufficient delay after the other has turned off. during turn-off of the lower mosfet, the phase voltage is monitored until it reaches a - 0.3v/+0.8v trip point for a forward/reverse current, at which time t he ugate is released to rise. an auto-zero comparator is used to correct the r ds(on) drop in the phase volt age preventing false detection of the -0.3v phase level during r ds(on) conduction period. in the case of zero cu rrent, the ugate is released after 35ns delay of the lgat e dropping below 0.5v. during the phase detection, the dis turbance of lgate falling transition on the phase node is blanked out to prevent falsely tripping. once the phase is high, the advanced adaptive shoot-through circ uitry monitors the phase and ugate voltages during a pwm fa lling edge and the subsequent ugate turn-off. if e ither the ugate falls to less than 1.75v above the phase or the phase falls to less than +0.8v, the lgate is r eleased to turn on. internal bootstrap device all three integrated drivers feature an inte rnal bootstrap schottky diode. simply adding an external capacitor across the boot and phase pins compl etes the bootstrap circuit. the bootstrap function is als o designed to prevent the bootstrap capacitor from o vercharging due to the large negative swing at the phase n ode. this reduces voltage stress on the boot to phase pins. the bootstrap capacitor must have a maximum voltage rating above pvcc + 5v and its capacitance value can be chosen from equation 11: where q g1 is the amount of gate charge per upper mosfet at v gs1 gate-source voltage and n q1 is the number of control mosfets. the ? v boot_cap term is defined as the allowable droop in the rail of t he upper gate drive. figure 10 shows the boot capacitor ripple voltage as a function of boot capacitor value and total u pper mosfet gate charge. e/a fb ofs vcc gnd + - + - 0.5v 1.5v gnd r ofs r 1 vdiff isl6308 figure 8. positive offset output voltage programming vref v ofs + - i ofs e/a fb ofs vcc gnd + - + - 0.5v 1.5v vcc r ofs r 1 vdiff isl6308 figure 9. negative offset output voltage programming vref v ofs + - i ofs (eq. 9) r ofs 0.5 r 1 ? v offset -------------------------- = ? v offset -------------------------- = ? v boot_cap -------------------------------------- ? q gate q g1 pvcc ? v gs1 -------------------------------- - n q1 ? = (eq. 11)
isl6308 fn9208 rev 4.00 page 15 of 28 sep 30, 2008 gate drive voltage versatility the isl6308 provides the user f lexibility in choosing the gate drive voltage for efficien cy optimization. the controller ties the upper and lower drive rails together. simply applying a voltage from 5v up to 12v o n pvcc sets both gate drive rail voltages simultaneously. initialization prior to initialization, proper conditions must exist on the enll, vcc, pvcc and the ref0 and ref1 pins. when the conditions are met, t he controller begins soft-start. once the output voltage is within the p roper window of operation, the controller asserts pgood. enable and disable while in shutdown mode, the pwm outputs are held in a high-impedance state to assure the drivers remain off. the following input conditions must be met before the isl6308 is released from shutdown mode. 1. the bias voltage applied at vcc must reach the internal power-on reset (por) risi ng threshold. once this threshold is reached, proper operation of all aspects of the isl6308 is guaranteed. hysteresis between the rising and falling thresholds assu re that once enabled, the isl6308 will not inadvertently turn off unless the bias voltage drops substanti ally (see electrical specifications on page 5). 2. the voltage on enll must be above 0.66v. the en input allows for power sequencing be tween the controller bias voltage and another voltage rail. the enable comparator holds the isl6308 in shutdown until the voltage at enll rises above 0.66v. the enable comparator has 100mv of hysteresis to prevent bounce. 3. the driver bias voltage applied at the pvcc pins must reach the internal power-on reset (por) rising threshold. in order for the isl6308 to begin operation, pvcc1 is the only pin that is required to have a voltage applied that exceeds por. however, fo r 2 or 3-phase operation pvcc2 and pvcc3 must also exceed the por threshold. hysteresis between the rising and falling thresholds assure that once enabled, the isl6308 will not inadvertently turn off unless the pvcc bias voltage drops substantially (see electrica l specifications on page 5). when each of these conditions is true, the controller immediately begins the soft-start sequence. soft-start during soft-start, the dac voltag e ramps linearly from zero to the programmed level. the pwm signals remain in the high- impedance state until the cont roller detects that the ramping dac level has reached the output -voltage level. this protects the system against the large, negat ive inductor currents that would otherwise occur when starting with a pre-existing charge on the output as the controller attempted to regulate to zero volts at the beginning of the soft-start cycle. the output soft-start time, t ss , begins with a delay period equal to 64 switching cycles after the en ll has exceeded its por level, 50nc 20nc figure 10. bootstrap capacitance vs boot ripple voltage ? v boot_cap (v) c boot_cap ( f) 1.6 1.4 1.2 1.0 0.8 0.6 0.4 0.2 0.0 0.3 0.0 0.1 0.2 0.4 0.5 0.6 0.9 0.7 0.8 1.0 q gate = 100nc figure 11. power sequencing using threshold- sensitive enable (enll) function - + 0.66v external circuit isl6308 internal circuit enll +12v por circuit 10.7k ? 1.40k ? enable comparator soft-start and fault logic vcc pvcc1
isl6308 fn9208 rev 4.00 page 16 of 28 sep 30, 2008 followed by a linear ramp with a rate determined by the switching period, 1/f sw . for example, a regulator wit h 450khz switching frequency having ref voltage set to 1.2v has t ss equal to 3.55ms. a 100mv offset exists on the remote-sense amplifier at the beginning of soft-start and ramps to zero during the first 640 cycles of soft-start (704 cyc les following enable). this prevents the large inrush current that would otherwise occur should the output voltage start out with a slight negative bias. the isl6308 also has the ability to start-up into a pre-charged output as shown in figur e 12, without causing any unnecessary disturbance. the fb pin is monitored during soft-start, and should it be hi gher than the equivalent interna l ramping reference voltage, the output drives hold both mosfets off. once the internal ramping reference exceeds the fb pin potential, the output drives are enabled, allowing the output to ramp from the pre-charged level to the final leve l dictated by the reference setting. should the output be pre-charged to a level exceeding the reference setting, the output drives are enabled at the end of the soft-start period, leading to an abrupt correction in the output voltage down to the reference set level. fault monitoring and protection the isl6308 actively monitors ou tput voltage and current to detect fault conditions. faul t monitors trigger protective measures to prevent damage to the load. one common power-good indicator is provided for linking to external system monitors. the schematic in figure 13 outlines the interaction between the fault monitors and the power-good signal power-good signal the power-good pin (pgood) is an open-drain logic output that transitions high when the converter is operating after soft-start. pgood pulls low d uring shutdown and releases high after a successful soft-st art. pgood transitions low when an undervoltage, overvoltage, or overcurrent condition is detected or when t he controller is disabled by a reset from enll or por. if after an underv oltage or overvoltage event occurs the output returns to within under and overvoltage limits, pgood will return high. undervoltage detection the undervoltage threshold is set at 82% of the ref voltage. when the output voltage ( vsen-rgnd) is below the undervoltage threshold, pgood gets pulled low. no other action is taken by the controller. pgood will return high if th e output voltage rises above 85% of the ref voltage. t ss 64 dac + 1280 ? f sw -------------------------------------------- = (eq. 12) figure 12. soft-start waveforms for isl6308-based multi-phase converter enll (5v/div) v out (0.5v/div) gnd> t1 gnd> t2 t3 output precharged below dac level output precharged above dac level figure 13. power-good and protection circuitry - + dac + 150mv vsen - + 0.82 x dac ov uv pgood soft-start, fault and control logic - + oc - + isen iref isum icomp ocset r ocset + - v droop v ocset + - v ovp 100a isl6308 internal circuitry - + rgnd x1 - + +1v vdiff droop* * connect droop to iref to disable the droop feature.
isl6308 fn9208 rev 4.00 page 17 of 28 sep 30, 2008 overvoltage protection the isl6308 constantly monitor s the difference between the vsen and rgnd volt ages to detect if an overvoltage event occurs. before, and during soft -start, while t he dac/ref is ramping up, the overvoltage trip level is v ovp ; upon successful soft-start, the overv oltage trip level changes to v ovp_rise (typically ref+150mv, see table electrical specifications on page 5) . should an ovp event be triggered for either case, the threshold hysteresis is 100mv (typical) below its trip point , as sensed at the vsen pins. actions are taken by the isl6308a to protect the load when an overvoltage condition occurs, until the output voltage falls back within set limits. at the inception of an overvo ltage event, all lgate signals are commanded high, and the pg ood signal is driven low. this causes the controller to turn on the lower mosfets and pull the output voltage belo w a level that might cause damage to the load. t he lgate outputs remain high until vdiff falls to within the over voltage limits explained above. the isl6308 will continue to pr otect the load in this fashion as long as the overvolt age condition recurs. during an ovp even t, the ovp pin is pulled high and pgood is pulled low. in additi on to the ovp pin behavior, the normal control loop be havior commands all lgate signals high as soon as the output voltage strays higher than the set point, essentially protecting against ovp events long before the ovp pin is triggered . thusly, the ovp output is a duplicate method of protecti on, allowing the user to implement secondary measures of protectio n for cases where normal circuit behavior is insufficient, or an additional level of protection is desir ed. the isl6308 continues to protect the output in the fashi on described for as long as the overvoltage condition is presen t and bias is applied to the circuit. for designs where the output voltage is scaled with a resistor divider (as shown in figure 6), the highest dac reference should normally be se lected, in order to give the most useful fixe d ovp trip point during soft-start, as well as after soft-start, where the ovp trip is referenced to the dac. once an overvoltage condition ends the isl6308 continues normal operation and pgood returns high. pre-por overvoltage protection prior to pvcc and vcc exce eding their por levels, the isl6308 is designed to protect the load from any overvoltage events that may occur. this is accomplished by means of an internal 10k ? resistor tied from phase to lgate, which turns on the lower mosfet to control the output voltage until the overvoltage event ceases or the input power supply cuts off. for complete prote ction, the low side mosfet should have a gate thresho ld well below the maximum voltage rating of the l oad/microprocessor. in the event that during norma l operation the pvcc or vcc voltage falls back below the por threshold, the pre-por overvoltage protection circuit ry reactivates to protect from any more pre-por overvoltage events. open sense line protection in the case th at either of the remot e sense lines, vsen or gnd, become open, the isl6308 is designed to detect this and shut down the controller . this event is detected by monitoring the voltage on the iref pin, which is a local version of v out sensed at the output s of the inductors. if vsen or rgnd become opened, vdiff falls, causing the duty cycle to increase and the output voltage on iref to increase. if the voltage on ir ef exceeds vdiff+1v, the controller will shut down. once the volt age on iref falls below vdiff+1v, the isl6308 will restart at the beginning of soft-start. overcurrent protection the isl6308 detects overcurre nt events by comparing the droop voltage, v droop , to the ocset voltage, v ocset , as shown in figure 13. the droop v oltage, set by the external current sensing circuitry, is proportional to the output curren t as shown in equation 8. a c onstant 100a flows through r ocset , creating the ocset vol tage. when the droop voltage exceeds the ocset voltage, the overcurrent protection circuitry activates . since the droop voltage is proportional to the output current, the overcurrent trip level, i max , can be set by selectin g the proper value for r ocset , as shown in equation 13. once the output current exce eds the overcurrent trip level, v droop will exceed v ocset , and a comparator will trigger the converter to begin overcur rent protection procedures. at the beginning of overcurrent s hutdown, the controller turns off both upper and lower mo sfets. the system remains in this state for a period of 4096 switching cycles. if the controller is still enabled at the end of this wait period, it will attempt a soft-start (as shown in figure 14). if the fault remains, the trip-retry cycles will continue indefinitely until either the controller is disabled or the fault is cleared. note that the energy delivered during trip -retry cycling is much less than during full-load operat ion, so there is no thermal hazard. r ocset i max r comp dcr ?? 100 ? ar s ? ---------------------------------------------------------- = (eq. 13)
isl6308 fn9208 rev 4.00 page 18 of 28 sep 30, 2008 general design guide this design guide is intended to provide a high-level explanation of the steps necessa ry to create a multi-phase power converter. it is assumed that the reader is familiar with many of the basic skills and techniques referenced in the following. in addition to this guide, intersil provides complet e reference designs that include schematics, bills of materials, and example board layouts for many applications. power stages the first step in designing a multi-phase converter is to determine the number of p hases. this determination depends heavily on t he cost analysis which in turn depends on system constraints that differ from one design to the next. principally, the designer wi ll be concerned with whether components can be mounted on both sides of the circuit board, whether through-hole co mponents are permitted, the total board space available for power-supply circuitry, and the maximum amount of load current. generally speaking, the most economical soluti ons are those in which each phase handles between 25a and 30a. all surface-mount designs will tend towar d the lower end of this current range. if through-hole mosfets and in ductors can be used, higher per-phase currents are possi ble. in cases where board space is the limiting constr aint, current c an be pushed as high as 40a per phase, but these designs require heat sinks and forced air to cool the mo sfets, inductors and heat dissipating surfaces. mosfets the choice of mosfets depends on the current each mosfet will be required to conduct, the switching frequency, the capability of the mosfets to dissipate heat, and the availability and nature of heat sinking and air flow. lower mosfet pow er calculation the calculation for the approximate powe r loss in the lower mosfet can be simplified, since v irtually all of the loss in the lower mosfet is due to c urrent conducted through the channel resistance (r ds(on) ). in equation 14, i m is the maximum continuous output current, i pp is the peak-to-peak inductor current (see equati on 1), and d is the duty cycle (v out /v in ). an additional term can be ad ded to the lower-mosfet loss equation to account for additional loss accrued during the dead time when inductor current is flowing through the lower- mosfet body diode. this te rm is dependent on the diode forward voltage at i m , v d(on) , the switching frequency, f sw , and the length of dead times, t d1 and t d2 , at the beginning and the end of the lower-mosfet conduc tion interval respectively. the total maximum power dissipated in each lower mosfet is approximated by the summation of p low,1 and p low,2 . upper mosfet power calculation in addition to r ds(on) losses, a large portion of the upper mosfet losses are due to cu rrents conducted across the input voltage (v in ) during switching. since a substantially higher portion of the upper-mosfet losses are dependent on switching frequency, the power ca lculation is more complex. upper mosfet loss es can be divided into separate components involving the upper mosfet switching times, the lower-mosfet body-dio de reverse recovery charge, q rr , and the upper mosfet r ds(on) conduction loss. when the upper mosfet turns off, the lower mosfet does not conduct any port ion of the inducto r current until the voltage at the phase node fal ls below ground. once the lower mosfet begins conducting , the current in the upper mosfet falls to zero as the current in the lower mosfet ramps up to assume the full inductor current. in equation 16, the required time for this commutation is t 1 and the approximated associated power loss is p up,1 . at turn on, the upper mosfet begins to conduct and this transition occurs over a time t 2 . in equation 17, the approximate power loss is p up,2 . a third component involves the lower mosfet reverse recovery charge, q rr . since the inductor current has fully commutated to the upper m osfet before the lower mosfet body diode can recover all of q rr , it is conducted 0a 0v output current figure 14. overcurrent behavior in hiccup mode output voltage p low 1 ? r ds on ?? i m n ----- - ?? ?? ?? 2 1d C ?? ? i lpp , 2 1d C ?? ? 12 ------------------------------------ - + ? = (eq. 14) p low 2 ? v don ?? f sw i m n ----- - i pp 2 -------- - + ?? ?? t d1 ? i m n ----- - i pp 2 -------- - C ?? ?? t d2 ? + ?? = (eq. 15) p up 1 , v in i m n ----- - i pp 2 -------- - + ?? ?? t 1 2 ---- ?? ?? ?? f sw ??? ? (eq. 16) p up 2 , v in i m n ----- - i pp 2 -------- - C ?? ?? ?? t 2 2 ---- ?? ?? ?? f sw ??? ? (eq. 17)
isl6308 fn9208 rev 4.00 page 19 of 28 sep 30, 2008 through the upper m osfet across v in . the power dissipated as a result is p up,3 . finally, the resistive part of t he upper mosfet is given in equation 19 as p up,4 . the total power dissipated by the upper mosfet at full load can now be approximated as the summation of the results from equations 16, 17, 18 and 19. since the power equations depend on mosfet p arameters, choosing the correct mosfets can be an it erative process involving repetitive solutions to the l oss equations for different mosfets and different switching frequencies. package power dissipation when choosing mosfets it is important to consider the amount of power being dissipa ted in the inte grated drivers located in the controller. sin ce there are a t otal of three drivers in the controller pack age, the total power dissipated by all three drivers must b e less than the maximum allowable power dissipation for the qfn package. calculating the power dissipation in the drivers for a desired application is critical to ensure safe operation. exceeding the maximum allowable power dissipation level will push the ic beyond the maximum recomm ended operating junction temperature of +12 5c. the maximum allowable ic power dissipation for the 6x6 qfn package is approximately 4w at room temperature. see layout consider ations on page 25 for thermal transfer im provement suggestions. when designing the isl6308 in to an application, it is recommended that the followin g calculation is used to ensure safe operation at the desired frequency for the selected mosfets. the total gate drive power losses, p qg_tot , due to the gate char ge of mosfets and the integrated drivers internal cir cuitry and their corresponding average driver current can be estimated wit h equations 20 and 21, respectively. in equations 20 and 21, p qg_q1 is the total upper gate drive power loss and p qg_q2 is the total lower gate drive power loss; the gate charge (q g1 and q g2 ) is defined at the particular gate to sourc e drive voltage pvcc in the corresponding mosfet data sheet; i q is the driver total quiescent current with no lo ad at both drive outputs; n q1 and n q2 are the number of upper and lower mosfets per phase, respectively; n phase is the number of active phases. the i q* vcc product is the qui escent power of the controller without capacitive load and is typically 75mw at 300khz. the total gate drive power l osses are dissipated among the resistive components along th e transition path and in the bootstrap diode. the portion of t he total power dissipated in the controller itself is the power dissipated in the upper driv e path resistance, p dr_up , the lower drive path resistance, p dr_up , and in the boot strap diode, p boot . the rest of the power will be dissipated by the external gate resistors (r g1 and r g2 ) and the internal gate resistors (r gi1 and r gi2 ) of the mosfets. figures 15 and 16 show the ty pical upper and lower gate drives turn-on transition path. the total power p up 3 , v in q rr f sw ?? = (eq. 18) p up 4 , r ds on ?? d i m n ----- - ?? ?? ?? 2 i pp 2 12 --------- - + ?? ? (eq. 19) p qg_tot p qg_q1 p qg_q2 i q vcc ? ++ = (eq. 20) p qg_q1 3 2 -- - q g1 pvcc f sw n q1 n phase ?? ??? = p qg_q2 q g2 pvcc f sw n q2 n phase ???? = -- - q g1 n ? q1 ? q g2 n q2 ? + ?? ?? n phase ? f sw i q + ? = (eq. 21) figure 15. typical upper-gate drive turn-on path figure 16. typical lower-g ate drive turn-on path q1 d s g r gi1 r g1 boot r hi1 c ds c gs c gd r lo1 phase pvcc ugate pvcc q2 d s g r gi2 r g2 r hi2 c ds c gs c gd r lo2 lgate
isl6308 fn9208 rev 4.00 page 20 of 28 sep 30, 2008 dissipation in the controller itself, p dr , can be roughly estimated as: current balancing component selection the isl6308 senses the channel load current by sampling the voltage across the lower mosfet r ds(on) , as shown in figure 17. the isen pins ar e denoted isen1, isen2, and isen3. the resistors connected between these pins and the respective phase nodes dete rmine the gains in the channel current balance loop. select values for these re sistors based on the room temperature r ds(on) of the lower mosf ets; the full load operating current, i fl ; and the number of phases, n using equation 23. in certain circumstances, it may be necessary to adjust the value of one or more isen resis tors. when the components of one or more channels are inhibited from effectively dissipating their heat so that the affected channels run hotter than desired, choose new, smaller values of r isen for the affected phases (see channel current balance on page 10). choose r isen,2 in proportion to the desired decrease in temperature rise in order to cause proportio nally less current to flow in t he hotter phase. in equation 24, make sure that ? t 2 is the desired temperature rise above th e ambient temperature, and ? t 1 is the measured temperature rise above the ambient temperature. while a single adjustment according to equation 24 is usually sufficient, it may occasionally be necessary to adjust r isen two or more times to achieve optimal thermal balance between all channels. load line regulation component selection (dcr current sensing) for accurate load line regulation, the isl6308 senses the total output current by detecting t he voltage across the output inductor dcr of each channel (as described in load line (droop) regulation on page 13) . as figure 7 illustrates, an r-c network is required to accur ately sense the inductor dcr voltage and convert this information into a droop voltage, which is proportional to the total output current. choosing the components for this current sense network is a two step process. first, r comp and c comp must be chosen so that the time constant of this r comp -c comp network matches the time constant of the inductor l/dcr. then the resistor r s must be chosen to set the current sense network gain, obtaining the desired full load droop voltage. follow the steps be low to choose the component values for this r-c network. 1. choose an arbitrary value for c comp . the recommended value is 0.01f. 2. plug the inductor l and dcr component values, and the values for c comp chosen in steps 1, into equation 25 to calculate the value for r comp . 3. use the new value for r comp obtained from equation 25, as well as the desired full load current, i fl , full load droop voltage, v droop , and inductor dcr in equation 26 to calculate the value for r s . due to errors in the inductance or dcr it may be necessary to adjust the value of r comp to match the time constants correctly. the effects of time constant mismatch can be seen in the form of droop overshoo t or undershoo t during the initial load transient spike, as shown in figure 18. follow the steps below to ensure the r -c and inductor l/dcr time constants are matched accurately. p dr p dr_up p dr_low p boot i q vcc ? ?? +++ = (eq. 22) p dr_up r hi1 r hi1 r ext1 + -------------------------------------- r lo1 r lo1 r ext1 + ---------------------------------------- + ?? ?? ?? p qg_q1 3 --------------------- ? = p dr_low r hi2 r hi2 r ext2 + -------------------------------------- r lo2 r lo2 r ext2 + ---------------------------------------- + ?? ?? ?? p qg_q2 2 --------------------- ? = r ext1 r g1 r gi1 n q1 ------------- + = r ext2 r g2 r gi2 n q2 ------------- + = p boot p qg_q1 3 --------------------- = ?? 50 10 6 C ? ----------------------- i fl n ------- - ? = (eq. 23) figure 17. isl6308 internal and external current- sensing circuitry isen(n) r isen v in channel n upper mosfet channel n lower mosfet - + i l x r ds on ?? i l isl6308 r isen 2 , r isen ? t 2 ? t 1 ---------- ? = (eq. 24) r comp l dcr c comp ? --------------------------------------- = (eq. 25) r s i fl v droop ------------------------ - r comp dcr ?? = (eq. 26)
isl6308 fn9208 rev 4.00 page 21 of 28 sep 30, 2008 1. capture a transient event with the oscilloscope set to about l/dcr/2 (sec/div). for example, with l = 1h and dcr = 1m ? , set the oscillosc ope to 500s/div. 2. record ? v1 and ? v2 as shown in figure 18. 3. select a new value, r comp,2 , for the time constant resistor based on the original value, r comp,1 , using equation 27. 4. replace r comp with the new value and check to see that the error is corrected. repeat the procedure if necessary. after choosing a new value for r comp , it will most likely be necessary to adjust the value of r s to obtain the desired full load droop voltage. use equation 26 to obtain the new value for r s . compensation the two opposing goals of compensating the voltage regulator are stability and speed. depending on whether the regulator employs the optiona l load-line regulation as described in load line (droop) regulation on page 13, there are two distinct metho ds for achieving these goals. compensating the load-line regulated converter the load-line regulated converter behaves in a similar manner to a peak current mode controller because the two poles at the output filter l-c r esonant frequency split with th e introduction of current informati on into the control loop. the final location of these poles is determined by the system function, the gain of the curren t signal, and the value of the compensation components, r 2 and c 1 . since the system poles and zero are affected by the values of the components that are m eant to compensate them, the solution to the system equation becomes fairly complicated. fortunately, there is a simple approximation that comes very close to an optimal solution. treating the system as though it were a voltage-mode regulator , by compensating the l-c poles and the esr zero of the voltage mode approximation, yields a solution that is always stable with very close to idea l transient performance. the feedback resistor, r 1 , has already been chosen as outlined in load line regulation component selection (dcr current sensing) on page 20 . select a target bandwidth for the compensated system, f 0 . the target bandwidth must be large enough to assure adequate transient performance, but smaller than one third of the per-channel switching frequency. the values of the compensation components depend on the relationships of f 0 to the l-c double pole frequency and the esr zero frequency. for each of the following three, there is a separate set of equations for the compensation components. r comp 2 ? r comp 1 ? v 1 ? v 2 ? ---------- ? = (eq. 27) figure 18. time constant mismatch behavior ? v 1 v out i tran ? v 2 ? i figure 19. compensation configuration for load-line regulated isl6308 circuit isl6308 comp c 1 r 2 r 1 fb vdiff c 2 (optional) 1 2 ? lc ? ? --------------------------- f 0 > r 2 r 1 2 ? f 0 v osc lc ? ?? ? 0.66 v in ? ----------------------------------------------------------- - ? = c 1 0.66 v in ? 2 ? v osc r 1 f 0 ??? ------------------------------------------------ - = case 1: 1 2 ? lc ? ? --------------------------- f 0 1 2 ? c esr ?? --------------------------------- < ? r 2 r 1 v osc 2 ? ?? 2 f 0 2 lc ???? 0.66 v in ? --------------------------------------------------------------- - ? = c 1 0.66 v in ? 2 ? ?? 2 f 0 2 v osc r 1 lc ? ?? ? ? --------------------------------------------------------------- ---------------- - = case 2: (eq. 28)
isl6308 fn9208 rev 4.00 page 22 of 28 sep 30, 2008 in equation 28, l is the per-chan nel filter inductance divided by the number of active channel s; c is the sum total of all output capacitors; esr is the equivalent series resistance of the bulk output filter capacitance; and v pp is the peak-to-peak sawtooth signal amplitude as described in electrical specifications on page 5. once selected, the compensation values in equation 28 assure a stable converter with reasonable transient performance. in most cases, tr ansient performance can be improved by making adjustments to r 2 . slowly increase the value of r 2 while observing the transient performance on an oscilloscope until no further improvement is noted. normally, c 1 will not need adjustmen t. keep the value of c 1 from equation 28 unless some per formance issue is noted. the optional capacitor c 2 , is sometimes n eeded to bypass noise away from the pwm comparator (see figure 19). keep a position available for c 2 , and be prepared to install a high frequency capacitor of between 22pf and 150pf in case any leading edge jitter problem is noted. compensating the converter operating without load-line regulation the isl6308 multi-phase conver ter operating without load line regulation behaves in a si milar manner to a voltage mode controller. this section highlights the design consideration for a voltage-mode controller requiring external compensation. to address a br oad range of applications, a type-3 feedback network is recommended (see figure 20). figure 21 highlights the voltage-mode control loop for a synchronous-rectified buck c onverter, applicable, with a small number of adjustments, t o the multi-phase isl6308 circuit. the output voltage (v out ) is regulated to the reference voltage, vref, level. the erro r amplifier output (comp pin voltage) is compared with th e oscillator (o sc) modified sawtooth wave to provide a pul se-width modulated wave with an amplitude of v in at the phase node. the pwm wave is smoothed by the outp ut filter (l and c). the output filter capacitor banks equivalent seri es resistance is represented by the series resistor e. the modulator transfe r function is the small-signal transfer function of v out /v comp . this function is dominated by a dc gain, given by d max v in /v osc , and shaped by the output filter, with a double pole break frequency at f lc and a zero at f ce . for the purpose of this ana lysis, l and dcr represent the individual channel inductanc e and its dcr divided by 3 (equivalent parallel value of t he three output inductors), whil e c and esr represents the tota l output capacitance and its equivalent series resistance. the compensation net work consists of the error amplifier (internal to the isl63 08) and the external r 1 -r 3 , c 1 -c 3 components. the goal of the co mpensation network is to provide a closed loop transfer function with high 0db crossing frequency (f 0 ; typically 0.1 to 0.3 of f sw ) and adequate phase margin (better than 45). phas e margin is the difference f 0 1 2 ? c esr ?? --------------------------------- > r 2 r 1 2 ? f 0 v osc l ?? ? 0.66 v in esr ?? ----------------------------------------------- ? = c 2 0.66 v in esr c ?? ? 2 ? v osc r 1 f 0 l ???? -------------------------------------------------------------- - = case 3: figure 20. compensation configuration for non-load-line regulated isl6308 circuit isl6308 comp c 1 r 2 r 1 fb vdiff c 2 r 3 c 3 figure 21. voltage-mode buck converter compensation design - + e/a vref comp c 1 r 2 r 1 fb c 2 r 3 c 3 l c v in pwm circuit half-bridge drive oscillator esr external circuit isl6308 v out v osc dcr ugate phase lgate - + vdiff vsen rgnd f lc 1 2 ? lc ? ? --------------------------- = f ce 1 2 ? c esr ?? --------------------------------- = (eq. 29)
isl6308 fn9208 rev 4.00 page 23 of 28 sep 30, 2008 between the closed loop phase at f 0db and 180. the equations that follow relate the compensation networks poles, zeros and gain to the components (r 1 , r 2 , r 3 , c 1 , c 2 , and c 3 ) in figure 20 and 21. use the following guidelines for locating the poles and zeros o f the compensation network: 1. select a value for r 1 (1k ? to 5k ? , typically). calculate value for r 2 for desired converter bandwidth (f 0 ). if setting the output voltage to be equal to the reference set voltage as shown in figure 21, the design procedure can be followed as presented. however, when setting the output voltage via a resistor divider placed at the input of the differential amplifier (as shown in figure 6), in order to compensate for the att enuation introduced by the resistor divider, the obtained r 2 value needs be multiplied by a factor of (r p + r s )/r p . the remainder of the calculations remain unchanged, as long as the compensated r 2 value is used. 2. calculate c 1 such that f z1 is placed at a fraction of the f lc , at 0.1 to 0.75 of f lc (to adjust, change the 0.5 factor to desired number). the higher the quality factor of the output filter and/or the higher the ratio f ce /f lc , the lower the f z1 frequency (to maximize phase boost at f lc ). 3. calculate c 2 such that f p1 is placed at f ce . 4. calculate r 3 such that f z2 is placed at f lc . calculate c 3 such that f p2 is placed below f sw (typically, 0.5 to 1.0 times f sw ). f sw represents the per-channel switching frequency. change the numerical factor to reflect desired placement of this pol e. placement of f p2 lower in frequency helps reduce the gain of the compensation network at high frequency, in turn reducing the hf ripple component at the comp pin and minimizing resultant duty cycle jitter. it is recommended a mathemat ical model is used to plot the loop response. check the lo op gain against the error amplifiers open-loop gain. verify phase margin results and adjust as necessary. the follo wing equations describe the frequency response of the modulator (g mod ), feedback compensation (g fb ) and closed-loop response (g cl ): compensation break f requency equations figure 22 shows an asym ptotic plot of the dc/dc converters gain vs frequency. the actual modulator gain has a high gain peak dependent on the quality fa ctor (q) of the output filter, which is not shown. using the above guidelines should yield a com pensation gain similar to the curve plotted. the open loop error amplifier gain bounds the compensation gain. check the compensation gain at f p2 against the capabilities of the error amplifier. the closed loo p gain, g cl , is constructed on the log- log graph of figure 22 by adding the modulator gain, g mod (in db), to the feedback compensation gain, g fb (in db). this is equivalent to multiplying the modulator transfer function and the compensation transfer function and then plotting the resulting gain. a stable control loop has a gain crossing with close to a -20db/decade slope and a phas e margin greater than 45 degrees. include worst case component variations when determining phase margin. the mathematical model presented makes a number of approximations and is generally not accurate at fr equencies approaching or exceeding half the switching frequency. when designing compensation networks, select target crossover frequencies in the range of 10% to 30% of the per-channe l switching frequency, f sw . r 2 v osc r 1 f 0 ?? d max v in f lc ?? --------------------------------------------- = (eq. 30) c 1 1 2 ? r 2 0.5 f lc ?? ? ---------------------------------------------- - = (eq. 31) c 2 c 1 2 ? r 2 c 1 f ce 1 C ??? -------------------------------------------------------- = (eq. 32) c 3 1 2 ? r 3 0.7 f sw ?? ? ------------------------------------------------ - = r 3 r 1 f sw f lc ------------ 1 C --------------------- - = (eq. 33) g mod f ?? d max v in ? v osc ----------------------------- - 1sf ?? esr c ?? + 1sf ?? esr dcr + ?? c ?? s 2 f ?? lc ?? ++ --------------------------------------------------------------- -------------------------------------------- ? = g fb f ?? 1sf ?? r 2 c 1 ?? + sf ?? r 1 c 1 c 2 + ?? ?? ---------------------------------------------------- ? = 1sf ?? r 1 r 3 + ?? c 3 ?? + 1sf ?? r 3 c 3 ?? + ?? 1sf ?? r 2 c 1 c 2 ? c 1 c 2 + -------------------- - ?? ?? ?? ?? + ?? ?? ?? ? --------------------------------------------------------------- ---------------------------------------------------------- ? g cl f ?? g mod f ?? g fb f ?? ? = where s f ?? ? 2 ? fj ?? = (eq. 34) f z1 1 2 ? r 2 c 1 ?? ------------------------------ - = (eq. 35) f z2 1 2 ? r 1 r 3 + ?? c 3 ?? ------------------------------------------------- = (eq. 36) f p1 1 2 ? r 2 c 1 c 2 ? c 1 c 2 + -------------------- - ?? -------------------------------------------- - = (eq. 37) f p2 1 2 ? r 3 c 3 ?? ------------------------------ - = (eq. 38)
isl6308 fn9208 rev 4.00 page 24 of 28 sep 30, 2008 output filter design the output inductors and the output capacitor bank together to form a low-pass filter responsible for smoothing the pulsating voltage at the phase nodes. the output filter also must provide the transient ene rgy until the regulator can respond. because it has a lo w bandwidth compared to the switching frequency, the outpu t filter limits the system transient response. the output capacitors must supply or sink load current while the curre nt in the output inductors increases or decreases to meet the demand. in high-speed converters, the output capacitor bank is usually the most costly (and oft en the largest) part of the circuit. output filter design begins with minimizing the cost o f this part of the circuit. the critical load parameters in choosing the output ca pacitors are the maximum size of the load step, ? i, the load-current sle w rate, di/dt, and the maximum allowable output-volt age deviation under transient loading, ? v max . capacitors are characterized according to their capacitance, esr, and esl (equivalent series inductance). at the beginning of the load trans ient, the outpu t capacitors supply all of the tr ansient current. the output voltage will initially deviate by an amoun t approximated by the voltage drop across the esl. as the load current increases, the voltage drop across the esr increases linearly until the load current reaches its final value. the capacitors selected must have sufficiently low esl and esr so that the total output voltage deviation is less than the allowable maximum. neglecting the contri bution of inductor cu rrent and regulator response, the output voltage ini tially deviates by an amount as follows: the filter capacitor must have sufficiently low esl and esr so that ? v < ? v max . most capacitor solutions rely on a mixture of high frequency capacitors with relatively low capacitance in combination with bulk capacitors having h igh capacitance but limited high-frequency performance. minimizing the esl of the high-frequency capacitors allow s them to support the output voltage as the current increases. minimizing the esr of the bulk capacitors allows them to supply the increased current with less output voltage deviation. the esr of the bulk capacitors also creates the majority of the output-voltage ri pple. as the bulk capacitors sink and source the inductor ac ripple cu rrent (see interleaving on page 9 and equation 2), a volt age develops across the bulk capacitor esr equal to i c,pp (esr). thus, once the output capacitors are selected, the maximum allowable ripple voltage, v pp(max) , determines the lower limit on the inductance. since the capacitors are suppl ying a decreasing portion of the load current while the re gulator recovers from the transient, the capacitor volt age becomes slightly depleted. the output inductors must be capable of assuming the entire load current before the output voltage d ecreases more than ? v max . this places an upper l imit on inductance. equation 41 gives the upper lim it on l for the cases when the trailing edge of t he current transient causes a greater output-voltage deviation than the leading edge . equation 42 addresses the leading edge. normally, the trailing edge dictates the selecti on of l because duty cycles are usually less than 50%. nevertheless, b oth inequalities should be evaluated, and l should be sel ected based on the lower of the two results. in each equ ation, l is the per-channel inductance, c is the total out put capacitance, and n is the number of active channels. switching frequency there are a number of variabl es to consider when choosing the switching frequency, as there are considerable effects on the upper mosfet loss calcul ation. these effects are outlined in mosfets on page 18, and they establish the upper limit for the switching frequency. the lower limit is established by the requiremen t for fast transient response and small output-voltage ripple as outlined in output filter design on page 24. choose the lowest switching frequency that allows the regulator to m eet the transient-response requirements. 0 f p1 f z2 open loop e/a gain f z1 f p2 f lc f ce compensation gain gain frequency modulator gain figure 22. asymptotic bode plot of converter gain closed loop gain 20 r2 r1 ------- - ?? ?? log log log f 0 g mod g fb g cl 20 d max v ? in v osc --------------------------------- log ? v esl ?? di dt ---- - ? esr ??? i ? + ? (eq. 39) lesr ?? v in nv ? out C ?? ?? v out ? f sw v in v pp max ?? ?? --------------------------------------------------------------- ---- - ? ? (eq. 40) l 2ncv o ??? ? i ?? 2 --------------------------------- ? v max ? iesr ? ?? C ? ? (eq. 41) l 1.25 ?? nc ?? ? i ?? 2 --------------------------------- - ? v max ? i esr ? ?? C v in v o C ?? ?? ?? ? (eq. 42)
isl6308 fn9208 rev 4.00 page 25 of 28 sep 30, 2008 switching frequency is determi ned by the selection of the frequency-setting resistor, r fs . figure 23 and equation 43 are provided to assist in sele cting the correct value for r fs input capacitor selection the input capacitors are resp onsible for sourcing the ac component of the input curre nt flowing into the upper mosfets. their rms current cap acity must be sufficient to handle the ac component of the current drawn by the upper mosfets which is r elated to dut y cycle and the number of active phases. for a three-phase design, use figure 24 to determine the input-capacitor rm s current requirement set by the duty cycle, maximum sustai ned output current (i o ), and the ratio of the peak-to-peak inductor current (i l,pp ) to i o . select a bulk capacitor with a ripple current rating which will minimize the total number of input capacitors required to support the rms current calculated. the vo ltage rating of the capacitors should also be at least 1.25 times greater than the maximum input voltage. figures 25 and 2 6 provide the same input rms current information for two-phase and single-phase designs respectively. use the same approach for selecting the bulk capacitor type and number. low esl, high-frequency ceramic capacitors are needed in addition to the input bulk capacitors to suppress leading and falling edge voltage spikes. the s pikes result from the high current slew rate produced by the upper mosfet turn on and off. place them as clos e as possible to each upper mosfet drain to minimize boar d parasitics and maximize suppression. layout considerations mosfets switch very fast and efficiently. the speed with which the current transitions from one devic e to another causes voltage spikes across the interconnecting impedances and parasitic circuit elements. these voltage spikes can degrade efficiency, r adiate noise into the circuit r fs 10 10.61 1.035 f sw ?? log ? C ?? = (eq. 43) figure 23. r fs vs switching frequency 100k 200k 500k 1m 2m switching frequency (hz) r fs value (k ? ) 10 20 50 100 200 figure 24. normalized input-capacitor rms current for 3-phase converter duty cycle (v in/ v o ) 00.4 1.0 0.2 0.6 0.8 input-capacitor current (i rms/ i o ) 0.3 0.1 0 0.2 i l,pp = 0 i l,pp = 0.25 i o i l,pp = 0.5 i o i l,pp = 0.75 i o figure 25. normalized input-capacitor rms current for 2-phase converter 0.3 0.1 0 0.2 input-capacitor current (i rms/ i o ) 00.4 1.0 0.2 0.6 0.8 duty cycle (v in/ v o ) i l,pp = 0 i l,pp = 0.5 i o i l,pp = 0.75 i o figure 26. normalized input-capacitor rms current for single-phase converter 00.4 1.0 0.2 0.6 0.8 duty cycle (v in /v o ) input-capacitor current (i rms /i o ) 0.6 0.2 0 0.4 i l,pp = 0 i l,pp = 0.5 i o i l,pp = 0.75 i o
fn9208 rev 4.00 page 26 of 28 sep 30, 2008 isl6308 intersil products are manufactured, assembled and tested utilizing iso9001 quality systems as noted in the quality certifications found at www.intersil.com/en/suppor t/qualandreliability.html intersil products are sold by description on ly. intersil may modify the circuit design an d/or specifications of products at any time without notice, provided that such modification does not, in intersil's sole judgment, affect the form, fit or function of the product. accordingly, the reader is cautioned to verify that datasheets are current before placing orders. information fu rnished by intersil is believed to be accu rate and reliable. however, no responsib ility is assumed by intersil or its subsidiaries for its use; nor for any infrin gements of patents or other rights of third parties which may result from its use. no license is granted by implication or otherwise under any patent or patent rights of intersil or its subsidiaries. for information regarding intersil corporation and its products, see www.intersil.com for additional products, see www.intersil.com/en/products.html ? copyright intersil americas ll c 2005-2008. all rights reserved. all trademarks and registered trademarks are the property of their respective owners. and lead to device overvoltage stress. careful component layout and printed circuit design minimizes the voltage spikes in the converter. consider, as an example, the turnoff transition of the upper pwm mo sfet. prior to turnoff, the upper mosfet was carrying channel current. during the turnoff, current stops flowin g in the upper mosfet and is picked up by the lower mosfet. any inductance in the switched current path generates a large voltage spike during the switching interval. carefu l component sel ection, tight layout of the critical components, and short, wide circuit traces minimize the magni tude of voltage spikes. there are two sets of crit ical components in a dc/dc converter using a isl6308 controller. the power components are the most critica l because they switch large amounts of energy. next are small signal components that connect to sensitive nodes o r supply critical bypassing current and signal coupling. it is important to have a symmetr ical layout, pre ferably with the controller equidistantly located from the three power trains it controls. equally impor tant are the gate drive lines (ugate, lgate, phase): since they drive the power train mosfets using short, high curr ent pulses, it is important to size them as large and as short as possible to reduce their overall impedance and inductanc e. extra care should be given to the lgate traces in pa rticular since keeping the impedance and inductance o f these traces helps to significantly reduce the po ssibility of shoot-through. equidistant placement of the con troller to the three power trains also helps to keep these traces e qually short (equal impedances, resulting in simila r driving of both sets of mosfets). the power components should be placed first. locate the input capacitors close to the power switches. minimize the length of the connections between the input capacitors, c in , and the power switches. loca te the output inductors and output capacitors between t he mosfets and the load. locate the high-frequency decou pling capacitors (ceramic) as close as practicable to the decoupling target, making use of the shortest connection path s to any internal planes, such as vias to gnd immediately next, or even onto the capacitor solder pad. the critical small components include the bypa ss capacitors for vcc and pvcc. locate th e bypass capacitors, cbp, close to the device. it is espec ially important t o locate the components associated with the feedback circuit close to their respective controller pins , since they bel ong to a high impedance circuit loop, sensiti ve to emi pick-up. it is also important to place current sens e components close to their respective pins on the isl6308, including the risen resistors, rs, rcomp, ccomp. f or proper curr ent sharing route three separate symmetrical as possible traces from the corresponding phase n ode for each risen. a multi-layer printed circuit board is recommended. figure 27 shows the connections of the c ritical components for the converter. note that capacitors c xxin and c xxout could each represent numerous physical capacitors. dedicate one solid layer, usually the one u nderneath the co mponent side of the board, for a ground pl ane and make all critical component ground connections with vias to this layer. dedicate another solid layer as a power plane and break this plane into smaller islands of c ommon voltage levels. keep the metal runs from the ph ase terminal to inductor l out short. the power plane shoul d support the input power and output power nodes. use copper filled polygons on the top and bottom circuit layers for the phase nodes. use the remaining printed circuit layers for small signal wiring. the wiring traces from the ic t o the mosfets gates and sources should be si zed to carry at le ast one ampere of current (0.02 to 0.05).
isl6308 fn9208 rev 4.00 page 27 of 28 sep 30, 2008 via connection to ground plane island on power plane layer island on circuit plane layer key figure 27. printed circuit board power planes and islands heavy trace on circuit plane layer +12v +12v +12v load c boot1 c boot2 r isen1 r isen2 r isen3 c boot3 c bin1 c bin2 (c hfout ) c bout c hf1 c bin3 locate close to ic locate near load; (minimize connection path) locate near switching transistors; (minimize connection path) (minimize connection path) c hf2 c hf3 c comp r comp r s r s r s r ocset pgood vdiff fb comp vcc isen1 isl6308 ref1 fs ofst ref phase1 ugate1 boot1 lgate1 isen2 phase2 ugate2 boot2 lgate2 isen3 phase3 ugate3 boot3 lgate3 isum icomp iref vsen rgnd ocset ref0 +5v pvcc1 pvcc2 pvcc3 enll +12v gnd ovp 2ph 3ph dac droop c 1 r 2 c 2 r 1 c hf0 r ofst r ref c ref c sum c hf01 l out1 c hf02 l out2 l out3 c hf03 r fs
isl6308 fn9208 rev 4.00 page 28 of 28 sep 30, 2008 package outline drawing l40.6x6 40 lead quad flat no-lead plastic package rev 3, 10/06 located within the zone indicate d. the pin #1 indentifier may b e unless otherwise specified, tolerance : decimal 0.05 tiebar shown (if present) i s a non-functional feature. the configuration of the pin #1 identifier is optional, but mus t be between 0.15mm and 0.30mm from the terminal tip. dimension b applies to the metallized terminal and is measured dimensions in ( ) for reference only. dimensioning and tolerancing c onform to amse y14.5m-1994. 6. either a mold or mark feature. 3. 5. 4. 2. dimensions are in millimeters. 1. notes: (4x) 0.15 index area pin 1 a 6.00 b 6.00 31 36x 0.50 4.5 4x 40 pin #1 index area bottom view 40x 0 . 4 0 . 1 20 b 0.10 11 ma c 4 21 4 . 10 0 . 15 0 . 90 0 . 1 c seating plane base plane 0.08 0.10 see detail "x" c c 0 . 00 min. detail "x" 0 . 05 max. 0 . 2 ref c 5 side view 1 10 30 typical recommended land pattern ( 5 . 8 typ ) ( 4 . 10 ) ( 36x 0 . 5 ) ( 40x 0 . 23 ) ( 40x 0 . 6 ) 6 6 top view 0 . 23 +0 . 07 / -0 . 05

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